AU605769B2

AU605769B2 - Process for preparing a high strength sheet material

Info

Publication number: AU605769B2
Application number: AU74691/87A
Authority: AU
Inventors: Ole-Bendt Rasmussen
Original assignee: Individual
Current assignee: Individual
Priority date: 1982-03-26
Filing date: 1987-06-25
Publication date: 1991-01-24
Anticipated expiration: 2003-03-25
Also published as: RU2110407C1; FI84035B; RU2072919C1; JPS59500409A; ES520833A0; IN158740B; AU1378983A; CA1222935A; JPH068029B2; FI85245C; FI834329A0; IE60191B1; DE3382793T2; EP0090380B1; ATE126479T1; DK150793B; ES8500805A1; DK150793C; EP0090380A1; DE3382793D1

Description

COMMONWEALTH OF AUSTRALIA COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952-69 COMPLETE SPECIFICATION

(ORIGINAL)

Class Application Number: Lodged: Int. Class Complete Specification Lodged: Accepted: Published: Priority: Related Art: This document contains the lamendmnents made under Section 49 and is correct for printing.

Narne of Applicant: Address of Applicant: Actual Inventor: Address for Service: OLE-BENDT RASMUSSEN 23, Forchwaldstrasse, Switzerland CH-6318 Walchwil/Zug, OLE-BENDT RASMUSSEN EDWD. WATERS SONS, 50 QUEEN STREET, MELBOURNE, AUSTRALIA, 3000.

Complete Specification for the invention entitled: PROCESS FOR PREPARING A HIGH STRENGTH SHEET MATERIAL The following statement is a full description of this invention, including the best method of performing it known to 2 PROCESS FOR PREPARING A HIGH STRENGTH SHEET MATERIAL The present invention relates to a method of preparing a high strength sheet material and is divided from co-pending Australian Patent Application No. 13789/83. Further the present invention relates to a method of preparing a high strength sheet material comprising forming a laminate comprising at least two layers of a thermoplastic polymer material, each layer having a fibrillar grain structure providing a predominant direction of splittability in said layer, the layers being bonded to one another with the said predominant directions of splittability transverse to each other, and biaxially orienting the molecules of said layers to sfretch.ing the layers in substantially uniaxial steps.

The British patent specification No. 1,526,722 describes the manufacture of a laminate by a method comprising 15 #0 t extruding at least two layers, each consisting of a blend of polymers which are incompatible to such a degree that the °ooo° blend on solidification forms a dispersion of particles of one polymer in a polymeric matrix melt, attenuating each 0000 layer to obtain a fibrillar grain structure having a 0 $t2Opredominant direction of splittability after solidification into a film, bonding the two layers to one another with the said predominant directions transverse to one another and Sbiaxially stretching the solidifed laminate in Ssubstantially uniaxial steps, the stretching being conducted B at temperature sufficiently low to maintain the predominant direction of splittability in each layer.

The specification of British patent No. 1,526,724 S describes the manufacture of a laminate comprising at least S two films of a polymetic material by a method which 30 0 00 comprises pressing the film together along lines extending substantially in the longitudinal direction of the films and l simultaneously stretching the films in the transverse direction, thereby forming a laminate having a waved 35 configuration in its transverse direction.

The latter method may advantageously be utilized to bond the two layers together and to effect the transverse stretching of the laminate in the method described in the 3 British patent specification No. 1,526,722.

The present invention relates to an advantageous material composition which in particular exhibits a high low temperature performance and which is readily stabilized against ultra-violet light.

By the combination of polymers which chemically are so closely related and blend homogeneously in the melt but still, i.a. due to the different molecular weights, clearly segregate from each other on cooling, one obtains a particularly fine and regular grain of polymer consisting of highly crystalline and relatively stiff microfibrils in a less crystalline and softer matrix. This structure has been observed in an electron microscope after selectively dissolving the matrix material. As mentioned above the grain 'i:15 thus produced was particularly regular and the distance between adjacent fibrils (from centre to centre) was in the order of magnitude 1/10.000 mm (1/10 um). The regular and o. fine structure, and the good bonding between the stiffer 0oe0 fibrils and softer matrix is of importance as far as the 0. strength properties are concerned. The crystalline anture of 1 20 the soft matrix gives the material low tendency to cold-flow.

€t ,In accordance with the present invention there is CC provided a method of preparing a high strength sheet material comprising forming a laminate comprising at least two layers of a thermoplastic polymer blend comprising polyethylene, each layer having a fibrillar grain structure providing a prerDminant direction of splittability in each tt said layer, the layers being bonded to one another with the said predomant directions of splittability transverse to 9 30 each other, and biaxially orienting the molecules of said layers by stretching the layers in substantially uniaxial steps to convert the grain of polymer into a zig-zagging micropattern, characterized by said blend being composed of high molecular weight high density polyethylene and low density polyethylene having a significantly lower molecular weight, said low density polyethylene being selected from the group of co-polymers and/or branched polyethylenes which r I -4a) exhibit substantially the same or higher elongation at break than the said molecular weight high density polyethylene when tested at room temperature under slow stretching, b) are capable of distinctly segregating, while forming a distinct microphase, from said high molecular weight high density polyethylene on cooling of a molten homogeneous blend of the said components.

The term "High molecular weight high density polyethylene" ("HMHDPE") comprises HDPE having a melt flow index of about or lower than 0.2 according to ASTMD 1238, condition E.

The designation "lineaty low density polyethylene" or (LLDPE" refers to polyethylene which is branched in C cc_ controlled manner to achieve a high elongation at break.

s' t t15 This controlled branching, as known, can be established, tote either by a high-pressure polymerization by using a suitable Cf catalyst, or by copolymerization with a suitable branch-forming monomer, such as butene, pentene, hexene or octene, of which the latter is preferable in connection with the present invention.

cFThe characteristic of the low density polyethylene S. having a significc4rmy lower molecular weight than the molecular weight of the high density polyethylene is v essentially defined (referred to hereinafter as hereinbefore' defined) such that the difference in molecular weights is sufficient to cause segregation into two polymer phases when the molten melt-attenuated blend of polymers is cooled to solidify.

Advantageously, the polyethylene is selected so ;o 30 that its shrinkability at 100 0 C, in the oriented state, is higher than the corresponding shrinkability of the polypropylene.

J

4a Under these circumstances, a special morphology will result. This morphology is characterized by twists or waves on the polypropylene fibrils with a twist or wave length of the order of 1 micron which can be observed in a scanning electron microscope.

The mechanical state of this structure has some similarity with cement which is reinforced with prestretched iron.

The blending ratio between the HMHDPE and the LDPE (preferably LLDPE) may conveniently be in the range of from 25:75 to 75:25.

HMHDPE exhibits a high tendency to molecular melt orientation. Such melt orientation (except when weak) tgenerally has been found a drawback in connection with the t present invention. In this connection one must distinguish sco t i t L1-

I

tODCCI f 0001 aPI OC C

I

E ai Id i 3COr

((LT

r

LC

i st r re t I r c c j c between the morphological "orientation" (grain of polymer) which is essential in the present invention, and the molecular melt orientation, which i.a. reduces the elongation of break and thereby the energy absorption.

Therefore, it is advisable to use low air cooling at the exit of the extruder so that the molecular melt orientation can be practically minimized.

Further improvements in this respect, and other essential improvements, can be obtained when the blend further contains polypropylene of a molecular weight significantly lower than said high molecular weight high density polyethylene, During draw-down at the exit from the extrusion die the HMHDPE will be molecularly oriented and will thereby "carry" 15 the film, so that the polypropylene is protected against any strong molecular orientation, and after crystallization of the polypropylene the latter will 'carry' the film so that the HMHDPE has the opportunity to loose part of its molecular orientation again.

The ratio in. the blend between the polypropylene and the 20 HMHDPE LDPE can conveniently be in the range of between 0 and 70/30.

The blend may further contain minor amounts of an alloying agent, e.g. a copolymer of propylene and a 25 polyolefin with 4 or more carbon atoms.

A further embodimenmt relates to the manufacture of a laminate having properties which make the material particularly useful for the manufacture of heavy duty sacks, The method according to this aspect comprises forming a laminate comprising at least two layers of a thermoplastic polymer material, each layer having a fibrillar grain providing a predominant direction of splittability in each said layer, the layers being bonded to one another with the said predominant directions of splittability transverse to each other, and biaxially orienting the molecules of said 3 5 layers by stretching the layers in sibstantially unaxial steps, and the method is characterized in that the direction of splittability of each layer of said layers of the I tI *o C

I

6 laminate to be biaxially oriented forms an angle of between and 350 with the machine direction of the laminate.

A sack usually has a width which is much smaller than its length and has been made in such a manner that the machine direction of the thermoplastic film becomes the length direction of the sack. During filling of the sack and regular handling of the filled sack, the most important performance factor is the yield point in its longitudinal direction. In case the filled sack is dropped, the most important performance factors are tear propagation strength, puncture strength and impact strength, the latter under forces which mainly act in the transverse direction of the sack.

It might be assumed that a laminate in which the direction of splittability of each layer is lying relatively close to the machine direction would be weak because a rupture (formed by puncturing or snagging) might easily S propagate under the influence of these transversely acting forces.

cooo The fact, however, is that the opposite is true, namely an 20 that the laminate thus produced generally exhibits an advantageous tear propagation resistance in all direction, particularly in the directions which form an angle of 450 to t C the machine direction. The tear propagation resistance in the said directions has been found determining for the strength of a stitched seam in cross laminated film material, such as a sewn sack.

An additional advantage obtained relates to the formation of heat seals in the manufacture and/or closing of the sack.

cc C 30 C tWhile the laminate can readily be formed into a tube with a glued or heat-sealed longitudinal seam having overlapping edges and a relatively low peel strength is sufficient in such type of seam, it is complicated and 3 expensive to fold the material to form overlapping edges at the bottom and/or at the top of the sack. Consequently, there is an important practical need for a high-strength film which readily can be heat-sealed to itself to form 7 seams across its machine direction with a resulting high peel strength.

One measure in this connection is the choice of a V suitable surface layer on the laminate.

Another measure is the allowance of a substantial contraction of the laminate perpendicular to the seam that I is parallel to the length of the sack, so that the increased thickness can compensate for the loss of molecular orientation caused by the heat-sealing. At the same time it is essential to limit the contraction of the laminate parallel to the direction of the seam which is perpendicular to the length of the sack.

It has now been found that the melt orientation of the molecules produced in connection with the extrusion (as distinguished from the subsequent biaxial orientation at a much lower temperature) plays a very important role for the contraction during heat sealing, and that consequently the C C C use of relatively small angles between the machine direction and the directions of splittability (substantially coinciding with the direction of melt orientation) leads to 20 S substantially improved heat seals at the top and/or the bottom of the sack, in particular with respect to the impact actions when a filled sack is dropped.

In this embodiment a heat treatment subsequent to the biaxial stretching is also highly preferable and similar to what is described in connection with the first aspect of the K invention, the heat treatment should preferably be carried out under such conditions that at least 7% shrinkage of the 09 laminate takes place in at least its transverse direction and it should generally be aimed at a higher shrinkage, such S *30 30 as 12% or more.

The use of the special transverse stretching method in connection with the disclosed subject matter of the parent patent application No. 13789/83 is also advantageous. The stetch ratio should preferably not exceed 2.5:1 in any direction and the optimum ratio is usually between 1.3-1.9, depending on the final use of the laminate. These values refer to the state when shrinkage has taken place (if 8 shrinkage has been effected).

Furthermore, this embodiment may advantageously be used in connection with the production of laminates produced in accordance with claims 22 and 23 of the above mentioned British patent specification No. 1.526.722. According to these claims one unoriented two-ply laminate in which the layers exhibit criss-crossing directions of splittability is produced directly by co-extrusion, using rotating die-parts.

It has now been found that the properties of the final biaxailly oriented laminate produced on the basis of this coextrusion method is substantially improved wh2n the angles between the directions of splittability of each layer and the machine direction fall within the range 10°-35 0 Finally, a further embodiment of this disclosure concerns the conservation of the correct amount and kind of 1 bending between the cross-laminated films, even after a a c oo; strong heat-treatment.

S' b The method according to this embodiment comprises 9:E forming a laminate from at lea.t two separately extruded films of thermoplastic polymer material, each film comprising a) a main layer exhibiting a fibrillar grain structure providing a predominant direction of splittability I in each said film, and b) a second layer for controlling bonding strength, the films being bonded to one another with the said predominant directions of splittability transverse to each other, the second layer of one film facing the second layer of the other film, and biaxially orienting the molecules of said layers by stretching the layers in substantially uniaxial steps, the transverse stretching and the bonding being effected by applying pressure to the 0 o surface of the laminate along lines extending substantially in the longitudinal direction of the laminate to impart thereto a waved configuration, and in which main layers the fibrillar grain structure consists of highly crystalline polypropylene and/or high density polyethylene micro fibrils which are generally embedded in a matrix material predominantly consisting of low density polyethylene, and preferably a) said matrix material exhibits an elongation at 9 break similar to or higher than that of the fibrilforming polypropylene or high density polyethylene when tested at room temperature under slow stretch, and b) said second layers mainly consist of branched polyethylene and exhibit a heat-seal temperature higher than 100 0 C and an elongation at break similar to or higher than that of fibrilforming polypropylene or high density polyethylene.

The branched polyethylene for the second layer preferably is LLDPE, to which there should usually be added about 35% or less of an elastomer, such as ethylene-propylene rubber. The matrix material may also conveniently be based on LLDPE.

When the laminate so produced is heated up to about 100 0 C either by a regular heat treatment as described e.g.

as disclosed in application No. 13789/83, or because hot goods hot cement) is packed in the laminate or in sacks made therefrom, the bonding strength will be ,maintained at the correct, not too high level, which is essential for the achievement of a high tear-propagation 20 .resistance.

a c 20 1It is preferred to heat treat the laminate below the temperature at which the second layers heat-seal to each other. The matrix material preferably has a melting range lower than that of said second layer and the laminate is allowed to shrink, at least in one direction, during said heat treatment.

The extruded films from which the laminate is produced may further comprise a surface layer which facilitates S sealing of the laminate. The said layer can with advantage t consist of plain or almost plain lineary low density S 30 polyethylene.

There is also disclosed herein apapratus for carrying out the method as disclosed in application No. 13789/83. The apparatus comprises means for preparing a laminate and means for stretching said laminate in substantially uniaxial steps, the means for transversely stretching the laminate comprising means for applying pressure to the surface of the laminate along lines extending substantially in the 1 i r vlongitudinal direction of the laminate to impart thereto a waved configuration, and the characteristic feature of said apparatus is that it further comprises means for heat treating the biaxially oriented laminate while allowing a Sshrinkage of the laminate to take place in at least its transverse direction.

The means for preparing the laminate are preferably Sthose described in British patent specification No.

1.526.722 and the means for transversely stretching the 10 laminate are preferably those disclosed in British patent specification No. 1.526.724.

As mentioned above the heating means preferably consist of a heated roller and the apparatus of the inveation preferably comprises means for inLroducing the laminate onto said heated roller in a longitudinally pleated configuration.

The latter means may be a separate pleating device but ~C 20 in case a waved configuration has been imparted to the laminate by means of a pair of intermeshing grooved rollers of the type described in British patent specification No.

1.526.724, it is preferred to place the heated roller in E close proximity of the surface of one of the rollers of said i pair of intermeshing grooved rollers to allow the laminate to be contacted with the surface of said heated roller immediately after having left the surface of said roller of the pair of intermeshing grooved rollers.

In this manner the fine waved configuration of the laminate is maintained during its travel from the pair of S 3 intermeshing rollers to the heated roller and the contracted laminate resulting from the following heat treatment S, 30 S 30 exhibits very useful strength properties.

In an apparatus in which the means for transversely stretching the laminate comprise at least one pair of intermeshing 11 grooved rollers, one or more conveyor rollers are preferably arranged between the last pair of intermeshing grooved rollers and the said heated roller, the adjacent rollers in said assembly being in so close proximity to one another that the film is supported by a roller surlace during substantially all of its travel from the last pair of grooved rollers to the heated roller.

The invention will be further described with reference to Fig. 1 of the drawings which schematically illustrates an apparatus for effecting the transverse stretching and heat treatment steps of the method of the invention.

Fig. 1 illustrates a rcil 1 of a laminate 2, and 3 is a set of grooved rollers. The set of grooved rollers 3 are mounted adjacent to an oscillating roller 4 mounted so close to a hot roller 5 that the laminate 2 is pressed against the surface of the hot roller 5 during shaft intervals. A cooling roller 6 is also mounted adjacent to the heated roller. The apparatus further comprises a set oi take-off ru lic.

and a roll 8. of transversely stretchez and !-eat treated §ni The operation of the apparatus illustrated is as follovws: Laminate 2 is unwound from the roll 1 and is passed through the nip of the set of grooved rollers 3 in which the laminate is stretched in its transverse direction so as to impart thereto a waved configuration. Following the transverse stretching the laminate is contacted with the oscillating roller 4 and subsequently contacted witl the hot roller 5. Due to the oscillating movement of the roller 4 relative to the hot roller 5 the heated laminate is free to shrink longitudinally. After leaving the hot roller 5 the laminate is cooled or, cooling roller 6 and is subsequently wound so as to form a fo!i after having passed through the nip of the set of take-off rollers 7.

The invention will now be described in further detail vit reference to the following examples.

EXAMPLE

A series of 3-layered tubular films are (extruded. Each lii, has a main layer in the middle, a layer for improved heat sealing oI one surface and a layer for improved lamination on the other surface.

The three layers form 75%, 15 9 and 10%, respectively, -f the ltct: film.

The main layer consists of i blend intirmately pre-biendec in a planetary screw extruder) of 1) a socalled "block-copolymer" of propylene and ethyiene sold under, the trade name "Hostalen 1022", 2) an ethylene-propylene rubber sold under- the trade name "Nordel 1500", 3) a high molecular weight high density polyethylene sol~d Under the trade name "Hlostaleri 9255 F".

Component 1I has melt Iflow, indlex ot* 0. 4 according to ASTNI D 1238 condi Lion Land analysis sliuws thatl it, contains about homo-polypropylene, MboutI 10I, pol yethiylene and about IQ0% ethylenepropylene rubber. A true block-copolymer is hardly detectable by the analysis, butL it is very likely that there are(- undetectable segments of* polyethylene on) the polypropylene which segments, assist 'in formnIinq a good polym-ier-in-polymei' dispersion.

Component 2 contains about ethylene and exhibits some ethylene crystallinit'v and a melt indlex of about. 0.3 measured at .190 00 but otherx'.SC ieunde!- 11t same conditions as k, the above mrentioned t about 0.05 Me'aSU."&O Under th e s amte conditions as omponert, 2.

Mhe blending ratios appear front the follow-,ing table 1, Samr- Codo 1. and r e, a "copo P-r True

PP

EPR.

admixed True EPR !ABLE I HRIIW

IIDPE.

admixed Final stretch ratio (after shrinkage, if shrunk) Annealing t emperature Shrink ratio 7.

DCD C IID CD Z. [245, i 9 72 j 10 7219 0 9 1,50: 1 1,50:1 245,90 1 38:1 1.5: 80 16 d.

f.

i.

38:1 80 1 247,1 247, 30r 255.1 255,-Ml 957,1 257. Q0 258,1 4-1.1 ett 754 75 51 1.44:1 1,56:1 Hi U,1 4_ 4 F- 4 -3 1,44:1 1,36:1 I 80 12 52 10 16 25 52 10 16 25 4_5 10 10 45 S1,46:1 1,5: -2 1,36:1 1,36:1 s8 16 15 '0 1754:1 1,50:1 r 1.34:1 80 18 21 -0 1,60:1 1,60:1 4 4 -45 14 258,', 255,1 450

U

e t 45 36 10 14 {45 1,42:1 1,4L:1 18 I 16 I I I -I I 3 25 1 6/:1 1 6 1 2 5 5 ,83 65 52 10 T 16 25 450 -v TIn. 255,5 10 65 52 10 16 25 255, 0 65 52 10 16 25 32i T '2' 1,46:1 1 ,40:1 _71 i 1,60:1 1,50:1 50 8 9,6 0.

P.

255, 7( i 179,1 med LLDPE 65 52 10 16 25 80 64 0 8 LLDPE i_ 1,56:1 1,46:1 60 12 11 2 1,40:1 1,40:1 70 16 14,6 1,56:1 1,50:1 I I f I 2 179,80 med LLDPE

LLDPE

20 1,40:1 1,40:1 12,5 4 1 1 4-2 255,802 4- 4 25 ca. ca.

1,40:1 I ,40:1 ca.16 ca. 3 S fle,~ j.

TABLE7'1: CcintOniudd) Weight Yield point in New- Energy at break, Ultimate tensile strength Elongation at ton Newtua x m in Newton break, g/sq.m. MD CD MD CD MD CD MD CD 71 27 12 14,9 8,9 89 67 554 421 82 36 26 15,9 10,1 85 55 563 538 76 34 14 17,8 10,9 98 71 596 507 87 36 26 18,0 13,5 87 63 637 641 32 12 21,0 7,3 101 54 706 410 98 39 28 17,9 14,7 84 65 636 686 74 27 14 15,8 8,5 8. 57 585 467 9 26 27 13,3 7,3 5? 50 450 391 Q6 39 19 23,1 12,7 125 80 658 497 26, 1 14,1 1 1.

h7 29 9 12,2 6,0 81 88 33 25 20,5 12,2 8f 75 31 16 14,5 8,0 P 79 31 19 19,9 9,9 9 92 33 23 18,2 10,6 8 83 40 20 19,8 11,8 12' 100 49 38 14,8 12,3 9 ca.100 33 32 18,0 10,7 7 502 780 392 588 51 577 470 3 -7 53 703 588 86 541 422 77 439 448 62 7L4 478 The layer for improved heat sealing consists of 70% "Hostalen 1022" and 30% "Nordel 1500".

The layer for improved lamination *consists of 50% "Hostalen 1022" and 50% "Nordel 1500".

The extrusion temperature is 250 C and the blow ratio 1:1.

Each of the tubular films is cut helically under an angle of 300 and two such films, each having a width of about 20 cm, are laminated and stretched with the layers for improved lamination facing one another. Initially, the lamination and simultaneous transverse stretching are effected by passing the films six or seven times through the nip between a set of grooved rollers of the type shown in British patent specification No. 1.526.722, Fig. 7. The division on each roller is 1.8 mm, the width of each tip is 0.4 mm and the tip is circularly rounded. The intermeshing between the tips is 0.9 mm. The stretch- S 15 ing is carried out at 350C.

Subsequently, each sample is stretched longitudinally at the same temperature by means of rollers.

Stretch ratios are determined by printed marks.

During the longitudinal stretching, the width is reduced significantly.

Those samples (as will 'be described below) whici are sub- K t jected to heat treatment are over-stretched in the longitudinal direc- 1 tion and finally further stretched in the transverse direction. The aim is that the heat treated samples should end at the same stretch ratios t 25 and square meter weight as those which are not heat treated. The pleated configuration created by this last transverse stretching is maintained in the film.

Heat treatment is then carried out at various temperatures on 60 cm long and 10 cm wide specimens which are carried forward and backward over a reciprocating heated roller during a period of 120 sec. and under a tension of 300 g. Different temperatures are tried. The specimens are brought in contact with the roller while they still have the pleated configuration but the pleats gradually disappear while the material shrinks.

Samples k and I deviate from the above by being cut under an angle of 450 instead of 300 Samples i and j deviate in being 4-layered. The angles are as follows: +450, +300, -300, -450 Samples p and q deviate by also being 4-layered materials, with the 'same directions and further by the composition of the main layer, which is: "Hostalen 1022" 20% linear low density polyethylene of melt index 1.0 and a density of 0.92.

The melt index is measured according to ASTM D 1238 condition L except that the temperature is 190 C, Sample r is a 2-ply sample similar to sample f regarding composition, angles and heat-treatment temperature, but deviates by not being subjected to the last transverse stretching and therefore not being in a pleated configuration when it is brought in contact with the hot roller. It is heat treated without any essential transverse j 15 contraction, but with longitudinal contraction similar to sample f.

Cj C 15 mm samples are cut in the machine and cross machine S' directions of each sample.

t e SStress-strain diagrams are taken at a velocity of 15 cm per I minute and an initial distance of 50 mm between the clamps.

The results obtained will appear from the table and from iI the diagrams in Figures 2 and 3. The diagrams in Fig. 2 "compare Sc samples e, f, m and o which all have the same composition and which are treated in the same manner, except that the annealing temperature j varies.

The diagrams in Fig. 3 compare samples b, d, f and h :j which contain different percentages of polyethylene, but otherwise are j 'i 'identical, the annealing temper'iLure bf this series being 80°C. In the I diagrams of Figures 1 and 2 the values of force and energy are corrected to a gauge of 80 g/m.

As regards the comparison between the sample r which, in essence, was not allowed to shrink transversely, and the similar sample f, which was allowed a significant shrinkage, the table shows that the shrunk film has essentially higher transverse elongation at break and transverse energy absorption, while the two samples have about the same yield point in the transverse direction.

EXAMPLE 2 The procedure described in example 1 is carried out on a number of film compositions, described in table 2 below, however with the last transverse stretching step and the subsequent heat-treatment

-C-

-Y

17 taking place in continuous manner on a pilot machine. During this stretching step, the intermeshing of the grooved rollers with each other is adjusted to obtain such a degree of pleating that there will be practically no transverse tension in the film during the heat treatment, but also so that all.pleats produced by this stretching disappear due to the transverse shrinkage.

The extrusion temperature is in all cases 200 0 C with a blow ratio of 1:1 and a moderate air cooling.

The high-strength laminate are in all cases made from two spirally cut extruded tubular films. Different angles of cutting have been tried, see table 2.

All steps of stretching are carried out at 35 0 C, and the heat treatment is effected on a roller heated to 80 0 C. The heat treatj ment Lakes about 10 seconds. The laminate is held practically tensione 15 free while being fed in between the last pair of grooved rollers (those Swhich im,,ediately preceed the roller for heat treatment). This measure causes the lan-inate to shrink about 5-10% in the longitudinal direction during the transverse stretching between the grooved rollers. After .t this stretching, the laminate follows the surface of one of these rollers and is then directly transferred from this surface to the surface of the hot roller, the distance between these surfaces being only about 1 cm. This guided transfer secures that the fine pleats, *C produced by the stretching between the grooved rollers, remain fine and even so as to cause an even transverse contraction on the hot roller.

The latter is driven at a circumferential velocity which is "about 10% lower than the circumferential velocity of the last set of i o grooved rollers. This measure, and a minimum tension at the take-off from the hot roller, gives the laminate a high freedom to shrink longitudinally.

\then leaving the hot roller, the laminate is transferred to a cooling roller, after which it is wound up.

The longitudinal and transverse stretch ratios are measured after each step of the process by measuring the deformation of circles, which have been printed on the film before the first stretching step. The aim is a final stretch ratio after the heat treatment) of 1.40:1 in both directions.

The adjustment of the transverse ratio takes place by the number of transverse stretching steps, which have been varied be- 18 tween 5 and 7 (to which comes the last one before the heat treatment). The adjustment of the longutidinal stretch ratio takes place by variation of the relative velocities of the rollers in the unit for longitudinal stretching. A proper adjustment of the stretch ratios is a complicated matter, and .variations between 1.35:1 and 1.45:1 have been tolerated.

The different laminates thus produced are tested for: a) Elmendorf Tear Propagation Resistance according to BS 308 B (43 mm tear), b) Beach Puncture Resistance according to BS 4816:72, c) Falling Dart Impact Strength according to ASTM 1709.

Description of the raw materials: The melt flow index refers to ASTM D 1238 condi- 15 tion L (in case of polypropylenes) or condition E (in the case of poletn leres or EFDM).

"Dc.lex 204.5": LLDPE cf density 0,920 and m.f..

I

It I Itt tIc "Hostalen 9255": "Hostalen 1050": "Hostalen 1022": "Novolen 1300 E": HMHDPE of density about 0,95 and m.f.i. about 0.05.

-homo-,PP of m.f.i. 0.4.

co-PP of m.f.i. 0.4 (further description see example 1).

gas phase-polymerized PP with about contents of atactic PP, partly forming a block-copoiymer with the isotactic

PP.

EPDM of m.f.i. about 0.3.

An EVA containing about 20% vinylacetate and of m.f.i. about "Nordel 1500":

EVA:

-19- Table 2 8 i

VI-.

601 I t a S I T 4 r ft Ir S l r

I

4 I L Compos it ion Film Inner layer (for Outer layer (for Code improved lamina- Middle layer sealing), 15% of No. tion), 10% of total 75% of total total R402 70% "Dowlex 2045" 80% "Hostalen 1022" 100% "Dowlex 2045" "Nordel 1500" 20% "Dowlex 2045" R404 80% "Novolen 1300 E" "Dowlex 2045" "Hostalen 9255" R407 35% "Hostalen 1022" 30% "Dowlex 2045" 50% "Hostalen 1022" 70% "Hostalen 20% "Hostalen 9255" 1022" "Nordel R414 20% "Dowlex 2045" 1500" 10% "Nordel 1500" "Novolen 1300 E" "Hostalen 1022" R417 20% "Dowlex 2045" 10% "Nordel 1500" "Hostalen 1050" "Hostalen 9255" R419 20% "Dowlex 2045" "Nordel 1500" R420 100% "Dowlex 2045" 60% "Hostalen 1022" R421 20% "Hostalen 9255"

"EVA"

"Hostalen 1022" "Hostalen 9255" R422 20% "Dowlex 2045" "Nordel 1500" Low density polyethylene (200 vm) Ordinary sack quality film for comparison 00.4*4* 19a- Table 2 (continued) 9 T C Ct 0 C C a t C Film Direc- Falling Elmendorf Beach Punc- Code tion Film Dart tear strength ture Resist- No. of ply w.eight Impact (43 mm. tear) ance, Joules degrees g/sq mn Strength,0 MD CD 450 MD CD R402 30 75 500-800 2160+ 1070 1340 11.0 9.4 75 600-800 1480 1100 920 14.2 13.3 R404 30 71 600-700 2910+ 1450 1770+ 6.4 9.1 74 600-800 2790+ 1830 1280 9.9 13.8 78 400-700 1620 1870 240,0+ 11..1 R4 07 45 85 400-600 3030+ 2110 2270+ 7.9 30 79 600-900 1410 1430 221 13.7 11.9 73 600-800 1460 1400 990 11.9 12.4 R414 60 71 600-800 1660 2750+ 1050 10.3 (average of I I IMD and CD) 77 600-900 1280 1250 2040+ 11.9 12.6 R417 45 82 800-900 1990 1460 750 13.3 14.0 60 80 800-900 1780 2490+ 650 12.1 13.6 74 500-700 .1670 1520 2050 10.2 R419 45 71 500-800 2020 1120 1360 11.3 10.5 R420 45 75 500-800 2450 1620 1850 -8.7 7.4 45 77 700-900 500 2190+ 800 12.9 9.9 R4 21 R422 45 84 700-900 2460+ 1420 2160+ 13.6 10.2 184 500-600 840 1300 1700 (average of MD and CD) o t

CCC

~C

I I

CC

OCIC

0 04 0 000009 0 0 0.94*04 0 0 higher than, exceeded the and indicates that one or more of the single tests maximum of the apparatus.

Several of the samples were further tested for Elmendorf Tear Propagation Strength at -15 0 C. For the samples of composition R 407, R 414 and f 419, this gave the same results (within the ranges of accuracy of this method) as the lest results at 20 0 C shown in table 2. This high performance.at low temperatures is surprising in view of the high contents of polypropylene, but explicable by the microstructure, which comprises the microscopical to submicroscopical fibrils of stiff polypropylene which are almost entirely embedded in relatively soft polyethylene.

A study of the tear resistance values in relation to the lamination angles (see table 2) gives the result that the 450 laminates show a significant weakness (relatively speaking) in their 450 directions, i.e. parallel to the direction of grain in one of the layers The same is not true for the 300 laminates, which generally show significantly higher all-over tear values, considering that the weakest direction generally determines the value of the laminate with I -spect to tear propagation resistance.

An exception to the rule that the 450 laminates exhibit a relatively low tear propagation resistance along the 450 direction, is found in the composition R 407. The main layer (middle layer) of this composition consists of HMHDPE and LLDPE in combination with a PP of significantly lower molecular weight than the HMHDPE, cf. claim 18. It is believed that the improved 450 tear strength in this case is due to the advantageous effects explained in the general description in connection with this claim.

Finally, the compositions containing 100% LLDPE in the laers for sealing R 402, 404, 407, 420, 421, 422) have'been found to form an adequate seal by ultrasonic sealing. The seal resists shear forces up to about 5-6 kp/2.5 mm and peel forces up to about 2 3f kp/ 2.5 mm. In this connection it is of importance that the seal layer and the matri\ in the middle layer consist of essentially the same material, namely both of a low-density polyethylene type, while the fibrillar, discontinuous, embedded phase of the middle layer consists of the much higher melting polypropylene.

EXAMPLE 3 High-strength. laminates were produced from two compositioris, both entirely consisting of HMHDPE and LLDPE, except for minor amounts of EPDM in the layer for improved lamination. The procedure was identical to that explained in example 2, except that a 21 prototype machine for full technical scale operation was used.

In both cases, the extrusion temperature was 2400C, the angle of cutting 450, the temperature of stretching 35 0 C, the temperature of the rollers for heat treatment 80 0 C, the time of heat treatment about 10 sec. Two heated rollers were used, one after the other, and subsequently two cooling rollers. The final stretch ratio, measured after heat treatment, was about 1.4:1 in both directions.

The entire stretching/lamination process including the heat treatment was operated in-line, the line comprising five transverse stretching stations, one longitudinal stretching station, and the last stretching station supplying the laminate with pleats for the "freeshrinkage" heat treatment. Between the last pair of grooved rollers and the first roller for heat treatment, and in close proximity to both, was an idle roller serving to keep the pleats fine and even.

The transverse stretching ratio was controlled by adjustrme nt of the internieshing between the grooved rolers in each of the Sfrst :fiv pa rs of grooved rDiers.

As in example 2, the intermeshing between the last pair of grooved rollers was adjusted to minimize' the transverse tension during the heat treatment.

The lineary velocity of the laminate at the exit from the stretching/lamination line was about 30 m/min.

The composition of the films and the results of the laboratory testing appear from table 3.

The polymer designations and the test methods for impact, tear and puncture resistance are explained in example 2 above, The other mechanical properties were determined from strain/stress cOrves taken for 15 mm wide specimens, the initial distance between the draw-clambs being 50 mm.

Strain/stress curves were taken as a modestly low velocity, namely 150 mm/min. and at a very low velocity, namely 15 mm/min.

The latter was tried in order to study the creep strength.

The yield tension (in Newton/mm 2 therefore was determined at each of the two velocities, while elongation at break (in and ul- 2 timate tensile tension (in Nevwton/ mm were determined only a. the velocity 150 mm/min.

The laminate prepared from composition R 1 was further converted to open-mouth sacks on commercial sack-making machinery.

It was first folded to a flat tube while being side-seamed by use of a

I_

22 commercial hot-melt adhesive, then cut into lengths while being heatsealed transversely to form the bottom of the sack. This seam was made by simple impulse sealing (without any kind of folding or overtaping) but with the conditions of sealing optimized to allow maximum shrinkage in the longitudinal direction. The dimension of the sack was about 100 cm x 50 cm. About 30 of such sacks were filled, closed by overtaping and drop-tested at minus 200C in competition with sacks of similar size made Irom a 185 g/sq n low density polyethylene film of standard quality for sack production. By these tests the high-strength laminate was found to be clearly superior in spite of its much lower gauge. The weight of the high-strength laminate used for these bag tests was 80 g/sq m, in other words almost 2k times as light as the ordinary polyethylene sack material.

t t 4 t ft t r i

I

r -23a- Table 3 (continued) c artri r Irri r ri d It t t tir (lil ~0*I

I)

((t 01 O t O tl t 0( ,1 Y19~eC O 1

J

u*rar~ o k in -23- Table 3 b j r 0 C dD o a o a O o 0 0 00 0ii, Com pos it ion Film Inner layer Middle Outer lay- Film Elmendorf Code for im- layer er (for weight Tear Strength No. proved la- improved mination) sealing) (43 mm tear) of to- 75% of to- 15% of total tal tal g/sq m MD CD 450 70% "Dow- 50% "Holex 2045" stalen 100% "Dow- 74 2020+ 2360+ 1350 9255" lex 2045" R1 "Nor-. 50% "Dowdel 1500" lex 2045" 70% "Hostalen 9255" R2 73 3200+ 3000+ 2220+ "Dowl ex 2045" higher than, and indicates that one or more of the single tests exceeded the maximum of the apparatus.

I certify that tIs 3rd yhe ?oaesr' 3 pafjs t zx and ex act copy of is 1 of 4 %eclficatrer or' rA y lodgd a.

S 0s

Claims

1. A method of preparing a high strength sheet material comprising forming a laminate comprising at least two layers of a thermoplastic polymer blend comprising polyethylene, each layer having a fibrillar grain structure providing a predominant direction of splittability in each said layer, the layers being bonded to one another with the said predominant directions of splittability transverse to each other, and biaxially orienting the molecules of said I layers by 4 stretching the layers in substantially uniaxial i steps to convert the grain of polymer into a zig-zagging micropattern, characterized by said blend being composed of high molecular weight high density polyethylene and low density polyethylene having a significantly lower molecular (as. n<"eixine-e de-uFed) weigh, said low density polyethylene being selected from the group of co-polymers and/or !branched polyethylenes which a) exhibit substantially the same or higher elongation at break than the said high molecular weight high density polyethylene when tested at room temperature under slow stretching, b) are capable of distinctly segregating, while forming a distinct microphase, from said high molecular weight high density polyethylene on cooling of a molten homogeneous blend of the said components. I

2. A method according to claim 1, characterized in that said blend further contains polypropylene of a Smolecular weight significantly lower than that of said high molecular weight high density polyethylene. j

3. A method according to claim 2, characterized in that said blend further contains minor amounts of an alloying agent. Q.g. a .c-polymcr of propylInC and an olofin with- 4 or morec crbon atoms. i &A4/i 25

4. A method according to claim 1 or 2, characterized by subjecting the sheet to shrinkage by at least 7% in at least one direction.

A method according to claim 1 or 2, characterized in that the stretch ratio in any direction and determined after shrinkage does not exceed 2.5:1.

6. A method as in claim 1, characterized in that the direction of splittability in each layer of said layers of the laminate to be biaxially oriented forms an angle of between 10 and 350 with the machine direction of the laminate.

7. A method as in claim i, characterized in that the stretch ratio in any direction and determined after shrinkage is between 1.3:1 and 1.9:1.

8. A method as in claim i, characterized in that the longitudinal stretching determined after shrinkage is effected at a stretch ratio of at least 105% of that of the transverse stretching determined after shrinkage.

9. A method as in claim 1, characterized in subjecting the biaxially oriented laminate to a heat treatment while allowing at least 7% shrinkage of the laminate to take place in at least its transverse direction.

A method as in claim 9, characterized in effecting the heat treatment by contacting a longitudinally pleated laminate with a heated roller.

11. A method as in claim 10, characterized in introducing onto the heated roller a laminate having the configuration obtained during the last transverse stretching step. Ot1< T 1 1 -26

12. A method according to claim 11, characterized in the laminate being allowed to contact longitudinally during said last transverse stretching step.

13. A method as in claim 1, wherein each film as a) a main layer exhibiting a fibrillar grain structure providing a predominant direction of splittability in each said film and b) a second layer for controlling bonding strength, the second layer of one film facing the second layer of the other film, characterized in that the transverse stretching and -the bonding are effected by applying pressure to the surface of the laminate along lines extending substantially in the longitudinal direction of the laminate to impart thereto a waved configuration.

14. A method as in claim 13, characterized in that the tI I i fibrillar grain structure consists of highly crystalline polypropylene and/or high density polyethylene micro fibrils which are .generally embedded in a matrix material S predominantly consisting of low density polyethylene. tat

15. A method as in claim 13, characterized in a) that said matrix material exhibits an elongation at break similar to or higher than that of the fibril forming polypropylene when tested at room temperature under slow stretch, b) that said second layers mainly consist of branched polyethylene and exhibit a heat-seal temperature higher than 100 0 C and an elongation at break similar to or higher than that of the fibril forming polypropylene or high density polyethylene.

16. A method according to claim 15, characterized by heat treating the laminate below the temperature at which the second layers heat-seal to each other. -27

17. A method according to claim 16, characterized in that said i that of said second layer and the laminate is allowed to shrink, at least in one direction, during said heat treatment.

18. A method according to claim 3 in which the alloying agent is a co-polymer of propylene and an olefin having 4 or more carbon atoms. DATED THIS 23 DAY OF JUNE 1987 OLE-BENDT RASMUSSEN I EDWD. WATERS SONS PATENT ATTORNEYS 50 QUEEN STREET MELBOURNE VICTORIA AUSTRALIA ;(2/39:SKP:DBM:AML) S 4 SII St c